I. Field of the Invention
This invention relates generally to computer output terminals of the type using an endless loop band having characters thereon for being impacted by a hammer for printing the character upon a paper surface adjacent thereto. In particular, this invention relates to an apparatus for controlling the elevation of the endless loop printing band for controlling the vertical spacing between adjacent printed lines.
II. Description of the Prior Art
Various prior art systems have been utilized for locating the position of a moving element relative to a reference position. Brown in U.S. Pat. No. 3,431,425 discloses a sensing system using dual light emitting elements for generating an error signal which is used to control a two phase servo motor, which then energizes a hydraulic valve control positioning mechanism to realign the traveling printing web. This system uses AC signals throughout for controlling a servo motor as opposed to the use of DC torque motors or stepping motors. Rempert in U.S. Pat. No. 3,598,978 discloses a method and apparatus for locating a preferred position on a workpiece. The workpiece is moved first in one direction and then in the opposite direction to determine with the aid of a light sensing element the distances between the initial position of the light sensing element and two of the workpiece edges.
Bessonny and Bowen in U.S. Pat. No. 3,432,672 disclose an automatic web registration control for use in printing presses. The control system employs a pair of photocells spaced apart by the distance equal to the separation between a pair of indicia. Pulse generation circuits are coupled to the photocells for generating paired pulses of opposite polarity, with the duration of the pulses being different when the web is out of registration. Mechanical means are provided for adjusting the position of the web responsive to the pulserations. Schneider in U.S. Pat. No. 3,525,872 discloses a system for detecting the arrival of a register mark on material traveling along a longitudinal axis, and responsive thereto generates a first control signal for triggering the performance of a work function on the work piece at a point in registration with the registration with the reference point. Hall and Beddell in U.S. Pat. No. 3,956,632 disclose a control system utilizing dual photoelectric sensors spaced on opposite sides of a longitudinally moving conveyor. Control signals are generated responsive to a transverse movement of the conveyor for reorienting the guiding sensors. Scanlon in U.S. Pat. No. 3,395,285 discloses a pinhole detector which automatically inspects very thin sheets of metal for minute perforations.
In contrast with these analog prior art systems, the present invention utilizes two pairs of optical sensors located adjacent to at least one edge of the moving printing band. The optical sensors could be placed adjacent opposite edges of the printing band if the variation in the width of the printing band were small with respect to the allowed position variation. The analog output signals from these sensors are then digitized to be input compatible with a digital computer which compares the sensor signals with a predetermined logic pattern. The digital computer generates a digital correction signal which operates a digital step motor for moving the printing band in incremental steps to correct the positioning error.